As for your piston damage I think that someone broke the piston by jamming it,then started the saw,heard (crunching) noise,then stopped it and cleared out the debris.It does not look the the chunk broke off when it was running. As for the groove around the bottom of the skirt it is not commonly seen,may have been to retain oil,my 275 has it too.
As for the ports design-a lot of drilled holes,this design has been used in other older 2-strokes,the Tecumseh 2-strokes are a good example. Yes the rings aren't pinned because they can't catch in these small holes.
I'm guessing the port timing in your model is similar to the 275.
These motors are also unique in that they have a large channel on each side of the crankcase extending down to the bottom front of the crankcase to feed the transfer ports in the cylinder,as partially seen in one of your photos. This gives the crankcases in the saws very high volume,I think these motors must have the lowest crankcase compression ratio of any 2-stroke I've seen. For this reason I'm going to adapt a motorcycle reed valve assembly to mine,and it will give me some nice options for power mods too.
I'm wondering if the larger HL carb and intake block from the 275 would fit on your saw...The cylinder and ports design of these saws do offer some interesting mods possibilities.
As for the ignition I wouldn't worry about finding points(But I think some newer ones match) because the cheap electronic modules should work on these saws,I've got one to put on mine.
I hope you find a piston,you will love the sound of that beast running.:msp_thumbup: